Timing axis for cathode ray oscillograph



Oct. 30, 1934. P L, H VEI; Er A 1,978,461

TIMING AXIS FOR CATHODE RAY OSCILLOGRAPH Filed NOV. 25, 1933 FIG! I -17 I 5 l g 7 3 J I I. 33 3 9 {32 I 5 I l6 5 I 1 3f 23 I I I J I I l G so Q FIG.2

I FIG-.3

T U v n FIG.4 W V V F/G'5 INVENTORS PAUL L. HOOVER T EVAN D, KENNEDY ATTORNEY Patented Oct. 30, 1934 UNITED STATES 1,978,461 'rnumo AXIS Fon carnona an oscnaocaarn Paul L. Hoover,

Middlebush, and Evan 1).

Kennedy, New Brunswick, N. 1., assignorl to Endowment Foundation. New Brunswick, N. 1., a corporation of New Jersey Application November 25, 1933, Serial No. 699,680

11 Claims.

This invention relates to a circuit arrangement especially adapted for use in connection with a cathode ray oscillograph, whereby a suitable timing axis is provided. Such a circuit is usually referred to as a sweep" circuit.

So far as we are aware, the sweep" circuits so far proposed-and there have been a numberare limited as to their speed of operation, particularly those utilizing mechanical devices for charging and discharging a condenser which controls the operation of the sweep" circuit. Those types of sweep" circuits, using a vacuum tube oscillator and varying the charge on the grid condenser of the oscillator tube, also have their speed limitations because the grid of the ordinary tube will not pass sufiicient current to produce the desired result, and to obtain high-speed action from such arrangements, the oscillator tubes must be made very large and are hence expensive and cumbersome in use.

It is therefore the principal object of our invention to provide an arrangement which will give a timing axis for a cathode ray oscillograph that will operate at an exceedingly high speed; that is, of the order of one million cycles or more per second, as distinguished from present-day arrangements that will only handle frequencies around twenty to thirty thousand cycles per second.

Our invention will be best understood by reference to the annexed drawing, taken in connection with the following description. In the drawing:

Figure 1 illustrates a diagrammatic arrangement of our improved form of timing axis or "sweep" circuit.

Figure 2 is a curve showing the charging characteristics of one of the condensers used in our circuit.

Figure 3 is a characteristic curve of the current flowing through one of the resistance units shown in Figure 1.

Figure 4 is a voltage curve across the grid filament of two of the vacuum tubes shown in Figure 1.

Figure 5 is a curve showing the characteristic of the voltage applied to the circuit extending to the oscillograph.

In Figure 1, 1 is a transformer having its primary 2 connected by a switch 3 to the power leads 4. The secondary 5 of the transformer l is connected to rectifier tubes 6 and 7 which furnish current to the filter arrangement generally referred to by the numeral 8. A second transformer 9 has its primary 10 also connected to the power leads 4 by means of a switch 11. One of the secondaries 12 of the transformer 9 supplies current to the filament or heater of tube 13, while the secondary 14 of the transformer 9 supplies current to the filaments of tubes 15 and 16, the other remaining winding 17 of the transformer 9 being connected to the rectifiers 6 and 7.

Associated with the filter 8 is a voltage divider 18 having a tap 19 extending to one of the gridplate-screens 20 of the tube 13. Another tap 21 extends to the balancing resistances 22 and 23 of the tubes 15 and 16. A further tap 24 extends to a high resistance 25. Interposed between these various taps and the terminals of the voltage divider 18, are the condensers 44, 45, 46 and 47.

A resistance 26 is connected across grid 2'? of the tube 13 and one end of the voltage divider 18. The resistance 26 is fairly low, being of the order of several hundred ohms. A resistance 28 is connected to one extremity of the voltage divider 18 and to the plate 29 of the tube 15, and also to one side of a variable condenser 30, the other side of which is connected to the grid 2'1 of the tube 13. The resistance 28 is comparatively high, being of the order of one hundred thousand to several hundred thousand ohms. The grid 31 of tube 15 and the grid 32 of tube 16 are connected to the plate 33 of tube 13. Connected across the plate filament circuit of tube 16 is a variable condenser 34 and extending from the opposite terminals of said condenser, is a circuit 0 extending to the oscillograph.

35 is a constant current tube having its filament energized by a battery 36 through the switch 37. A battery 38 supplies voltage to one of the grids 39 of the tube 35, while the other grid 40 is connected to a potentiometer 41 connected by the switch 42 to a source of current such as battery 43. The potentiometer 41 thus regulates the bias on the control grid 40. It is to be understood that the sources of current 36 and 38 are adjusted so that the tube 35 passes a constant current, the magnitude of which can be controlled by the potentiometer 41.

In the operation of our circuit, when the source of alternating current is applied to the transformer 9 by closing the switch 11, current is sent through the filaments or heaters of the tubes 13, 15 and 16. On closing the switch 3, power is applied to the transformer 1, the output of which passes throughthe rectifier tubes 6 and 'l and the filter circuit 8, and is impressed across the voltage divider 18.

The potential taps 19, 21, 24 and 48 are so adjusted that the potential between the grid and filament of tubes 15 and 16 is highly negative, thereby preventing any current from flowing from the plate to the filament 01! these tubes until the plate voltage reaches several hundred volts. When power is applied as has been described, current flows through resistance 25 and tube 13. Current also flows through resistances 28 and 26, charging condenser 30. Because of the characteristics 01' the resistances as previously described, the charging of condenser 30 does not take place instantaneously, but follows a curve similar to that portion of the curve from the point 49 to 50 of Figure 2.

It is thus seen that the current through resistance 26 is not constant but will follow a curve similar to that shown in Figure 3, where current I is plotted against time T. While the current through resistance 26 is relatively large, there is a positive potential applied to the grid of tube 13. As this current decreases, this positive bias decreases and this in turn results in a decrease in the plate current through tube 13 which is also the current through resistance 25.

A relatively small change in the current through resistance 25 causes a large positive shift in the potential of the grids 31 and 32 of tubes 15 and 16. As the grid 01 tube 15 becomes less negative, or stated in another way, more positive, a part of the current through resistance 28 will be passed by the plate circuit of the tube 15 and thus less current will fiow through condenser 30 and through resistance 26. This in turn results in a still lower or less positive potential on the grid of the tube 13, and a further reduction in current through resistance 26, a still greater reduction in current through resistance 25, and a still greater reduction of current through the tube 15. and so on. The accumulative efi'ect takes place extremely rapidly and finally results in condenser 30 discharging through the tube 15 and resistance 26. During the discharge cycle of the condenser 30, the current through resistance 26 reverses its direction as shown in Figure 3. As soon as condenser 30 is discharged, the conditions are back at the starting point and the whole cycle is repeated.

The wave form of the voltage across the grid filament of tubes 15 and 16 is shown in Figure 4. Except for a reversal of sign, this is similar to Figure 3. When the grid of tube 13 is negative, the grids of tubes 15 and 16 are positive and vice versa.

The two tubes 13 and 15 thus constitute an oscillator, the essential feature being that we get a wave form similar to that shown in Figure 4 to drive the grid of tube 16; that is to say, we desire a voltage that is negative for most of the cycle but rapidly swings to a high positive value and then rapidly returns. It is to be understood that when switches 37 and 42 are closed, a constant current fiows from the source of current supply through the tube 35 and charges condenser 34. The tube 16 has such a high initial negative bias that nocurrent flows through it, but as a result of the constant flow of current into the condenser 34, the voltage across this condenser will increase linearly with time as shown in Figure 5.

Periodically, however, due to the operation of the other part of the circuit as has been described, the grid of tube 16 will be driven highly positive for a short time and condenser 34 will be completely discharged through the tube 16, and then the cycle repeated. The voltage across the condenser 34 is taken oil by the circuit 0 to the deflection plates of the cathode ray oscillograph. It may be noted that for the purposes of stabilization, a small coupling condenser 51, connected to the circuit under investigation, is provided and serves to keep the sweep" circuit exactly in step with the wave form under examination.

From what has been said, it will be seen that the oscillator must give a wave form having a characteristic similar to Figure 4, or stated in another way, the oscillator must drive the grid of the hard discharge tube 16 highly positive at predetermined intervals; that is to say, the oscillator should give the desired wave form directly without any further amplification. Furthermore, during the charging period, the hard tube 16 must pass no current. Therefore the added requirement is that the oscillator must maintain the grid of the said discharge tube, highly negative during the charging interval and then rapidly drive the grid positive long enough to discharge the condenser directly connected to the oscillograph circuit, and then rapidly return to the high negative value.

We have shown the various portions of our sweep" circuit as enclosed in separate shields as indicated by the broken lines 52, as it is highly essential for extreme speeds in the oscillator to keep the stray capacities to a minimum. It may also be noted that in order to vary the sweep" time, the condensers 30, 34 and potentiometer 41 are made variable. The frequency of the cycle of operation may also be varied by changing the magnitude of the resistances 26 and 28, or the position of the various voltage taps i9, 21, 24 and 48.

What we claim is:

1. In a "sweep" circuit for the deflection plates of a cathode ray oscillograph, a condenser included in said circuit, means for charging said condenser from a source of current at a constant rate, a vacuum tube having at least three elements therein, its plate-filament circuit being connected across said condenser, means for maintaining the grid of said tube highly negative while said condenser is being charged, and means for rapidly driving the grid positive long enough to discharge the condenser through the tube as and for the purpose described.

2. In a sweep circuit for the deflection plates of a cathode ray osclllograph, a condenser included in said circuit, means for charging said condenser from a source of current at a constant rate, a vacuum tube having at least three elements therein, its plate-filament circuit being connected across said condenser, an oscillator circuit connected to the grid of said tube for maintaining the said grid highly negative during the charging interval of said condenser, and means in the oscillator circuit for rapidly driving the said grid positive to discharge the condenser at predetermined intervals.

3. A "sweep" circuit as set forth in claim 2, further characterized in that the oscillator circuit delivers to the grid of said tube without further amplification, a voltage having a highly peaked wave from.

4. A sweep circuit as set forth in claim 2, further characterized in that the oscillator circuit delivers to the grid of .said tube a voltage which is negative for most of the cycle of operation, but which swings very rapidly to a high positive value and then very rapidly returns.

5. In a sweep circuit for the deflection plates of a cathode ray oscillograph, a condenser included in said circuit, means for charging said condenser from a source of. unidirectional current at a constant rate, and means including, firstly a hard tube for discharging said condenser at a high rate of speed, and secondly, an oscillator delivering a peaked voltage directly to the grid of said hard tube for the control thereof.

6. A sweep" circuit as set forth in claim 5, further characterized in that said means includes an oscillator circuit connected to the grid of said hard tube, said oscillator circuit applying to the grid a voltage which is relatively highly negative while the said condenser is being charged and then swings very rapidly to a high positive value for the purpose described, and then swings very rapidly back to a relatively high negative value.

'7. A sweep" circuit as set forth in claim 2,

further characterized in that the oscillator circuit comprises; two vacuum tubes having the grid of one connected to the plate of the second through a condenser, a very, high resistance connected between a source of suitable current and the plate of said second tube and the condenser connected thereto, another; resistance of relatively low value connected between said source of suitable current and the grid of said one tube and condenser, and a third resistor connected between the plate and grid of said two tubes and to said source of suitable current intermediate the connections of said other two resistances mentioned.

8. A sweep circuit as set forth in claim 2, further characterized in that the oscillator circuit comprises two vacuum tubes interlinked by a condenser from the grid of one to the plate of the other, means for charging said condenser and means for maintaining the grid of the second tube in the oscillator circuit highly negative until the plate voltage of said tube reaches several hundred volts, and means including the first-mentioned tube in the oscillator circuit for shifting the voltage on the second tube of the oscillator circuit very rapidly to a relatively high positive value similarly as in the said tube connected directly to the oscillograph circuit.

9. A sweep circuit, for use with a cathode ray oscillograph, including a condenser connected across the circuit leading to the oscillograph, means comprising a constant current tube for applying an electric charge to said condenser at a constant rate, and means including a high vacuum electronic tube having at least three elements therein and responsive to ultra-high frequencies for discharging said condenser at predetermined intervals.

10. A sweep circuit, for use with a cathode ray oscillograph, including a condenser connected across the circuit leading to the oscillograph, means' comprising a constant current tube for applying an electric charge to said condenser at a constant rate, a vacuum tube having at least three elements and having its plate and filament connected across said condenser, means for initially maintaining the grid highly negative, an oscillator circuit connected to the grid of said tube, said oscillator including means for driving the grid of said tube very rapidly positive at required intervals to cause said condenser to discharge for the purpose described.

11. Means for supplying a timing axis for a cathode ray oscillograph including; a condenser adapted to be connected across the deflection plates of the oscillograph, means for periodically charging said condenser, an electronic tube having at least three elements, two of which, the plate and filament, being connected across said condenser, and an oscillator connected to the grid of said tube, said oscillator being capable of maintaining said grid highly negative while said condenser is being charged, and then very rapidly driving the said grid positive for a brief interval to discharge said condenser.

PAUL L. HOOVER. EVAN D. KENNEDY. 

